The present invention relates generally to the field of arrays of redundant array independent disk (RAID) data storage devices, and more particularly to utilizing a plurality of sets of data storage devices, each of the sets using different combinations of the data storage devices to improve rebuild performance.
RAID is used as an umbrella term for computer data storage configurations that divide user data and redundancy information among multiple data storage devices in an array of data storage devices. Redundancy information enables the reconstruction of contents of a failed data storage device, with or without taking the storage system offline. Redundancy information may be, for example, a mirror copy of the user data (for example, in RAID 1) or parity information (for example, in RAID 5). The mirror copy configuration may be cost prohibitive to implement since both the data and its duplicate are stored, needing twice the storage capacity. A mirroring configuration incurs a storage capacity overhead for redundancy of at least 100%. To provide a system that incurs a lower storage capacity overhead for redundancy, parity may be implemented.
In traditional RAID configurations, without declustering, when a data storage device in the array fails, all (or nearly all, for traditional RAID 6) of the surviving data storage devices in the array are read during a reconstruction/recovery of a single failed data storage device. This allows the configuration to rebuild the failed data storage device's content from the remaining data storage devices. If the system is not, or cannot, be taken offline during reconstruction, user throughput during reconstruction is reduced, and the surviving data storage devices bear an additional load by being read to reconstruct the failed data storage device's content while online. The additional load on the surviving data storage devices during an online rebuilding may slow down the recovery and thereby increase the time needed to perform reconstruction of the failed data storage device's content. A declustered RAID array may improve rebuilding performance.